There are several known techniques for growing solid crystal structures. The Czochralski process is among the most widely used of these techniques. In the Czochralski process, a heated crucible contains a liquid melt form of a charge material from which the crystal is to be grown. The melt is maintained at a temperature slightly above the solidification temperature of the charge material. During the crystal growth process, a crystal seed is placed at one end of a cable (or rod) and positioned in the melt material. Once positioned, the crystal seed is slowly raised out of the melt material (by the cable) and rotated about its longitudinal axis to form the crystal. The seed can be either a sample of the desired crystal material, or any other material that has a higher melting temperature and the same crystal structure as the melt material in its solid form. When the seed is lowered into the melt material, it causes a local decrease in melt temperature that results in a portion of the melt material crystallizing around and below the seed. Thereafter, the seed is slowly withdrawn from the melt and into a crystal growth chamber. As the seed is withdrawn, the portion of the newly formed crystal that remains within the melt essentially acts an extension of the seed and causes melt material to crystallize around and below it. This process continues as the crystal is withdrawn from the melt, resulting in further crystal growth as the seed is continually raised away from the melt.
The crystal is withdrawn from the growth chamber and into a transition chamber (located above the crucible) having a temperature lower than that of the crucible. The crystal finally is drawn from the transition chamber into an elongated receiving chamber shaped to accommodate the extended length of the crystal. The crystal typically is cooled by passing a cooled heat exchange fluid, such a water, through the walls of the transition chamber. This removes both heat from an inert gas surrounding the crystal, and radiant heat emitted from the crystal.
One problem that occurs during the crystal growth process is that crystal lattice defects (commonly referred to as "crystal dislocations") in the seed crystal can propagate throughout the growing crystal. It is known, however, that a dislocation-free silicon crystal may be grown using the Czochralski process if the crystal structure is formed to include a thin neck portion. Such a thin neck portion typically has a relatively small diameter of about 3 to 5 millimeters and must be grown from the seed crystal prior to forming a larger diameter crystal body. One problem with such a thin neck portion, however, is its limited load capacity. The weight of the crystal may exceed the load carrying limit of the crystal neck, thereby causing the crystal neck to break. This problem is particularly serious with large diameter crystals, such as 300-400 millimeter diameter crystals. For example, a crystal neck having a diameter of three millimeters generally can support a maximum crystal body weight of about 143 kilograms. However, a crystal with a 300 millimeter diameter body, for example, often weighs more than 200 kilograms. In such situation, the thin crystal neck of the heavy crystal can fracture, resulting in undesirable interruption of the crystal growing process.
Accordingly, it would be desirable to provide a mechanism for growing a crystal that does not rely on the relatively thin crystal neck to support the load of the entire crystal body. Un addition, it would be desirable to provide such a mechanism in a single, compact unit. Furthermore, it would be desirable to incorporate such a crystal growing mechanism into presently available Czochralski crystal pulling systems. Such mechanism would permit the formation of large diameter, defect free crystal that does not require substantial capital investment for new crystal pulling systems.